High sensitivity micro-machined pressure sensors and acoustic transducers

ABSTRACT

A method of making a pressure sensor or acoustic transducer having high sensitivity and reduced size. A thin sensing diaphragm is produced by growing a single crystal, highly doped silicon layer on a substrate using a chemical vapor deposition process. The diaphragm is incorporated into a pressure sensor or acoustic transducer which detects pressure variations by a change in the capacitance of a capacitor which includes the diaphragm as a movable member. The thin diaphragm produces a highly sensitive device which can be fabricated in a smaller size than sensors or transducers having thicker diaphragms.

This is a divisional of application Ser. No. 08/643,091, filed May 2,1996 now U.S. Pat. No. 5,888,845.

TECHNICAL FIELD

The present invention relates to pressure sensors and acoustictransducers used for microphones and other acoustic wave sensingdevices, and more specifically, to a method of making highly sensitiveand compact forms of such devices.

BACKGROUND OF THE INVENTION

Pressure sensors are used to detect variations in the pressure exertedon a surface or to measure the absolute value of the pressure exerted,and to convert the measured quantity into an electrical signal. A commonuse of such sensors is in the form of an acoustic transducer. Acoustictransducers detect changes in the pressure applied by sound waves andconvert such changes into a varying electrical signal. Acoustictransducers may also be used in a reverse sense to convert electricalsignals to pressure waves. When connected to an amplifier which drives asecond transducer which reproduces the applied sound wave, an acoustictransducer can be made to function as a microphone.

One common type of acoustic transducer utilizes a diaphragm which movesin response to an applied sound wave. The diaphragm forms one plate of atwo-plate capacitor. The movement of the diaphragm changes theseparation of the capacitor plates, causing a variation in thecapacitance of the capacitor. When used in conjunction with theappropriate circuitry, the change in capacitance produces an electricalsignal which is proportional to the applied pressure. It is well knownthat the sensitivity of such a pressure sensor increases with a decreasein the thickness of the diaphragm. This is because a thinner diaphragmhas less inertia and can respond more rapidly to small pressurevariations. In addition, because the size of the transducer scales withthe thickness of the diaphragm, reducing the thickness of the diaphragmleads to both a more sensitive and a smaller device.

FIG. 1 is a side view of a prior art micro-machined pressure sensor oracoustic transducer 100 formed by processing techniques used in thesemiconductor industry. As shown in the figure, transducer 100 is formedfrom a silicon substrate 102 into which is etched an aperture 104 forentrance of a pressure wave. The pressure wave impacts diaphragm 106,causing the diaphragm to move in response to variations in the pressureapplied by the wave. Diaphragm 106 forms one plate of a two-platecapacitor. The second plate is formed by a perforated electrode (notshown) located above diaphragm 106. Movement of diaphragm 106 causes thecapacitance of the two-plate capacitor to vary, producing a changingelectrical signal.

There are two primary methods currently used in the semiconductorindustry to fabricate diaphragm 106 for use in an acoustic transducer.The first method is based on the diffusion mechanisms which occur inboron gas phase doping processes. This method uses boron tri-chloride(BCl₃) as the source gas to produce a highly doped p+ layer which servesas an etch stop. While this method is capable of producing diaphragmfilms of thickness greater than one micron (1 μm), it has not proveduseful in producing thinner films. This is because it has not beenpossible to reproducibly grow highly doped films of thickness less thanone micron in this manner.

The second method used for producing diaphragms of the type shown inFIG. 1 is to implant Boron ions into a thin film. The use of such amethod in fabricating a differential pressure sensor is discussed inU.S. Pat. No. 5,332,469, entitled "Capacitive Surface MicromachinedDifferential Pressure Sensor", issued Jul. 26, 1994. However, such amethod of forming the diaphragm is not useful for fabricating very thindiaphragms because implantation of the Boron ions produces stress in thefilm which causes buckling and cracking of the diaphragm.

Thus, both of the currently used methods for making diaphragms forpressure sensors and acoustic transducers are incapable of reliablyproducing diaphragms having a thickness less than one micron. As aresult, the methods cannot be used to produce pressure sensors oracoustic transducers which have increased sensitivity and reduced sizecompared to currently available devices. Another disadvantage of the twocurrently used methods for forming the sensor diaphragm is that theyresult in the formation of a parasitic reverse biased p+ n diode(element 108 of FIG. 1) which acts to electrically isolate diaphragm 106from substrate 102. This increases the power required to operate thedevice and reduces the responsivity of the two-plate capacitor, makingthe sensor less sensitive.

What is desired is a method for producing pressure sensors and acoustictransducers having a high sensitivity and reduced size which overcomesthe disadvantages of currently used techniques.

SUMMARY OF THE INVENTION

The present invention is directed to a method of making a pressuresensor or acoustic transducer having high sensitivity and reduced size.The method is based on forming a thin sensing diaphragm by growing asingle crystal, highly doped silicon layer on a substrate using achemical vapor deposition process. The diaphragm is incorporated into apressure sensor or acoustic transducer which detects pressure variationsby a change in the capacitance of a capacitor which includes thediaphragm as a movable member. The thin diaphragm produces a highlysensitive transducer which can be incorporated into a smaller sizedevice than transducers having thicker diaphragms.

Further objects and advantages of the present invention will becomeapparent from the following detailed description and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a prior art micro-machined pressure sensor oracoustic transducer formed by processing techniques used in thesemiconductor industry.

FIGS. 2 through 7 show the process flow for fabricating a thin diaphragmfor use in a pressure sensor or acoustic transducer in accordance withthe present invention, using processing techniques used in thesemiconductor industry.

FIGS. 8 through 13 show the process flow for forming an acoustictransducer using the thin diaphragm produced according to the method ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated when discussing the prior art acoustic transducer of FIG.1, it is desirable to make the diaphragm of the sensor as thin aspossible in such devices. This accomplishes two purposes: (1) itincrease the sensitivity of the device; and (2) it allows the device tobe scaled down in size, thereby producing a smaller device. However,present methods of fabricating thin diaphragms have the disadvantagesnoted. The inventors of the present invention have overcome thesedisadvantages by the process flow which will now be described. Theresult is a highly sensitive pressure sensor or acoustic transducerwhich can be scaled down to smaller sizes than currently availabledevices.

FIGS. 2 through 7 show the process flow for fabricating a thin diaphragmfor use in a pressure sensor or acoustic transducer in accordance withthe present invention. As shown in FIG. 2, fabrication of the pressuresensor or acoustic transducer begins with a silicon substrate 200.Typically, substrate 200 is composed of N or P-type silicon having a<100> crystal orientation, and a resistivity of 5 to 10 ohm-cm. Ifsubstrate 200 is P-type, then the diaphragm is formed in an N well.Substrate 200 may be double polished to produce the proper surface forthe later processing steps. A 0.6 to 1 micron thick layer of field oxide204 is grown on the top 201 and bottom 202 sides of substrate 200.

Using standard photolithographic techniques known in the semiconductorindustry, a photoresist mask is used to pattern field oxide layer 204 ontop side 201 of substrate 200. A wet oxide etch process based on a BHFchemistry is used to remove portions of field oxide layer 204, producingthe oxide pattern shown in FIG. 2. It is noted that bottom side 202 ofsubstrate 200 is protected during the oxide layer etch on top side 201of the substrate. Regions 206 where the oxide has been removed from topside 201 will be processed further to form the locations for the devicecontacts.

After etching the oxide layer on the top of substrate 200, similarphotolithographic techniques are used to pattern field oxide layer 204on bottom side 202 of substrate 200. A wet oxide etch process based on aBHF chemistry is used to remove portions of field oxide layer 204,producing the oxide pattern shown in FIG. 2. It is noted that top side201 of substrate 200 is protected during the etch of the oxide layer onbottom side 202 of the substrate. The result of the two oxide etch stepsis the structure shown in FIG. 2.

Next, a deep Boron diffusion process is carried out using Boron Nitrideor BCl₃, at a temperature of 100 to 1150° C. This produces a deep p+Boron diffusion region 208 in regions 206 and the uncovered portions ofbottom side 202 of substrate 200. A photoresist layer (not shown) isthen spun onto top side 201 of substrate 200. This serves to protect topside 201 during the subsequent steps. An etch process is then carriedout to remove the remaining field oxide 204 on bottom side 202 ofsubstrate 200. The photoresist layer previously deposited onto top side201 of substrate 200 is then removed. The resulting structure is shownin FIG. 3.

A mask is then used to pattern oxide layer 204 on top side 201 ofsubstrate 200, followed by the removal of that portion of oxide layer204 covering the substrate where the diaphragm will be formed. As shownin FIG. 4, a silicon or silicon-germanium layer (elements 210 and 212)is then deposited on top side 201 of substrate 200. The deposited layeris used to form a thin, highly doped p+ layer which will serve as thediaphragm for the pressure sensor or transducer. The silicon orsilicon-germanium layer can be grown in a suitable single waferepitaxial deposition (chemical vapor deposition) tool, such as an ASMEpsilon E2 Reactor, manufactured by ASM of Phoenix, Ariz.

The silicon or silicon-germanium layer is doped with Boron ions to aconcentration of 1×10²⁰ ions/cm³, using B₂ H₆ as a Boron source gas andSiH₄ or SiH₂ Cl₂ as a silicon source gas. The deposition process isoptimized to cause a layer composed of single crystal silicon 210 to begrown on the regions of substrate 200 not covered by field oxide 204,while a layer of polysilicon 212 is grown on the regions covered byfield oxide 204. Polysilicon layer 212 will be used to provide anelectrical contact to the diaphragm formed from single crystal siliconlayer 210. Thus, a single process step permits the formation of both thethin diaphragm and a contact to that layer. This reduces the complexityof the process flow used to form the contact, which typically requiresadditional step(s) devoted to formation of a contact layer. Thedeposition tool allows control of the thickness of layer 210, producinga diaphragm having a thickness of between 0.02 to 1 microns.

As mentioned, highly doped p+ layer, which will be used as thediaphragm, can be composed of silicon or silicon-germanium. Ifsilicon-germanium is used, the strain in the deposited film layer can betailored by altering the germanium content of the film. This permits theresponsivity of the diaphragm to the applied pressure to be modified,and assists in controlling the buckling of the diaphragm. In addition,the heavy doping of the diaphragm layer causes the layer to act as anetch stop for the etching of the backside edge of the substrate (used toform an aperture to allow the pressure wave impact the diaphragm). Thiscombination of processes allows precise control of the thickness of thediaphragm.

A photolithographic mask is then used to define the desired extent ofpolysilicon layer 212. The remaining portion of layer 212 is removedusing a reactive ion etch (RIE) process in a Triode reactor using a Cl₂based chemistry. The resulting structure is shown in FIG. 5. Polysiliconlayer 212 will be used to form an isolated contact to layer 210, whichforms the sensor or transducer diaphragm.

A layer of low temperature oxide (LTO) 214 is then deposited on top ofsubstrate 200. LTO layer 214 is 0.25 to 0.7 microns thick and isdeposited by means of a chemical vapor deposition process. The resultingstructure is shown in FIG. 6.

A photolithographic mask is then used to define the portions of LTOlayer 214 corresponding to contact region 216 and single crystal region210. Those portions of LTO layer 214 are removed using a wet etchprocess based on a BHF chemistry. The resulting structure is shown inFIG. 7. As noted, contact region 216 will be used to form an electricalcontact to the diaphragm of the sensor.

The process flow up to this point has produced a thin, highly dopedsingle crystal silicon or silicon-germanium layer which serves as asensor diaphragm. This structure may then be subjected to furtherprocessing to produce a pressure sensor or acoustic transducer. If it isdesired to produce a pressure sensor, the backside of substrate 200 mustbe etched to provide an aperture for entry of the pressure wave.Substrate 200 may be etched from the backside by using a mask and an EDPetch process performed at approximately 105° C. A KOH or TMAH based etchmay also be used. The highly doped p+ regions 208 will not be etched,while the lower doped regions will be removed by the etch process. Thepressure sensor is then completed by forming a second contact to thedevice. This contact may be formed by bonding a second silicon substrateon top of the structure of FIG. 7. As diaphragm 210 of the sensor movesin response to an impacting pressure wave, it will alter the separationof the capacitor plates of the capacitor formed by the diaphragm and thesecond substrate. This will alter the capacitance of the capacitor,permitting sensing of the pressure wave. Another method of forming thesecond contact is to follow the process flow for forming the metal seedlayer or perforated electrode, which will be discussed with reference toFIGS. 11 and 12. Other methods compatible with the described processflow may also be used. After formation of the second contact, thepressure sensor structure is packaged and tested.

It is noted that due to the existence of the polysilicon layer on top ofthe oxide layer, the backside etch used to form the aperture for entryof the pressure wave is not as sensitive to the alignment of the etchmask as is the case for process flows found in the art. This is becauseover-etching in the lateral direction is not an issue as it would be foralternative process flows. This means that the thickness and size of thediaphragm can be more precisely controlled, producing both a moresensitive and compact device. In order to make the aperture etch processsufficiently insensitive to the etch mask alignment, it is suggestedthat the thickness of oxide layer 204 be substantially greater (e.g., afactor of five or so) than that of sensor membrane 210.

FIGS. 8 through 13 show the process flow for forming an acoustictransducer using the thin diaphragm produced according to the method ofthe present invention. Continuing the processing of the structure ofFIG. 7, a sacrificial photoresist or low temperature film layer 218 isthen deposited. This layer is patterned using photolithographictechniques. The photoresist layer is etched, producing the structureshown in FIG. 8. As shown in the figure, the photoresist or lowtemperature film remains over single crystal region 210. A metal seedlayer 220 is then sputtered onto the top of substrate 200. Layer 220 istypically composed of a gold-nickel-vanadium alloy, a titanium-nickelalloy, or silver. The resulting structure is shown in FIG. 8. Metal seedlayer 220 provides a base for formation of a second electrode for thetransducer.

A photoresist layer 222 is then spun onto substrate 200. Layer 222 isdeposited using multiple steps of spinning on the photoresist, until athickness of approximately 25 microns is obtained. A photolithographymask is used to define the plating region used to form a backplate orperforated bridge electrode for the transducer. The photoresist layer isthen selectively developed. The resulting structure is shown in FIG. 9.

A layer of gold or silver 224 of thickness approximately equal to thethickness of photoresist layer 222 is then plated onto substrate 200,filling the portions of photoresist layer 222 removed during theprevious process step. Layer 224 is typically applied by a pulsedplating process to a thickness of approximately 20 microns. The layer ofgold or silver 224 will be used to form the perforated bridge electrodewhich acts as one plate of the two-plate capacitor. The resultingstructure is shown in FIG. 10.

The remaining portions of photoresist layer 222 are then removed by asuitable resist removal solution. The portions of metal seed layer 220not covered by gold or silver layer 224 are then removed by a wet etchbased on a HNO₃ /HF chemistry. The resulting structure is shown in FIG.11.

The sacrificial layer of photoresist or low temperature film 218 is thenremoved by a suitable resist removal solution. This produces an openingbetween single crystal layer 210 and gold or silver layer 224. Theopening serves as the gap between the two plates of the capacitor formedfrom single crystal layer 210 and gold or silver layer 224. Theresulting structure is shown in FIG. 12.

Substrate 200 is then etched from the backside to produce an aperture226 which allows acoustic waves to enter the transducer and impact thediaphragm. The substrate etching is performed by an EDP etch processperformed at approximately 105° C. A KOH or TMAH based etch process mayalso be used. The highly doped p+ regions 208 will not be etched, whilethe lower doped regions will be removed by the etch process. Theresulting structure is shown in FIG. 13. The transducer structure isthen packaged and tested.

As shown in FIG. 13, the acoustic transducer 228 of the presentinvention includes a thin diaphragm 210 formed from a highly doped layerof single crystal silicon. Acoustic pressure waves enter transducer 228by means of aperture 226. The waves impact diaphragm 210, causing it tovibrate. This causes a displacement between the plates of a two-platecapacitor formed from diaphragm 210 and the perforated electrodecomposed of gold or silver layer 224. The displacement between theplates causes the capacitance to vary, producing a signal which can bemeasured. The change in capacitance is proportional to the magnitude ofthe applied pressure. Contact region 216 provides an isolated contactregion on polysilicon layer 212 to diaphragm 210 of the sensor. Thecontact region may be used as is, or be further processed by depositinga metal layer to form a contact.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed.

We claim:
 1. An acoustic transducer or pressure sensor, comprising:asubstrate having a top surface and a bottom surface; a single crystalsilicon diaphragm formed on the top surface of the substrate, thediaphragm having a thickness of one micron or less; an aperture formedin the bottom surface of the substrate and extending to the diaphragm;and a contact electrically connected to the diaphragm, the contactlocated on the top surface of the substrate and electrically isolatedfrom the substrate.
 2. The acoustic transducer or pressure sensor ofclaim 1, further comprising:a layer of polysilicon interposed betweenthe contact and the substrate.
 3. The acoustic transducer or pressuresensor of claim 2, further comprising:a layer of oxide interposedbetween the polysilicon layer and the substrate.
 4. The acoustictransducer or pressure sensor of claim 3, wherein the layer of oxide issubstantially thicker than the single crystal silicon diaphragm.
 5. Theacoustic transducer or pressure sensor of claim 1, wherein the singlecrystal silicon diaphragm is doped with a dopant.
 6. The acoustictransducer or pressure sensor of claim 1, wherein the single crystalsilicon diaphragm is a silicon-germanium layer.
 7. The acoustictransducer or pressure sensor of claim 1, further comprising:aperforated electrode above the single crystal silicon diaphragm, theperforated electrode being electrically isolated from the single crystalsilicon diaphragm and separated from the single crystal silicondiaphragm by a capacitor gap, thereby forming a two-plate capacitorcomprised of the single crystal silicon diaphragm and the perforatedelectrode.